1. Field of the Invention
The present invention relates to a voltage controlled oscillator that is suitably used for a cellular phone or the like.
2. Description of the Related Art
A conventional voltage controlled oscillator will be described with reference to the drawings. FIG. 13 is an exploded perspective view of the conventional voltage controlled oscillator, and FIG. 14 is a cross-sectional view of essential parts of the conventional voltage controlled oscillator.
Next, a configuration of the conventional voltage controlled oscillator will be described with reference to FIGS. 13 and 14. A circuit board 51 comprises a laminate of a plurality of thin insulating plates 52. On a surface 51a of the circuit board 51, a wiring pattern 53 made of a conductive pattern is provided. In addition, on an entire back surface 51b of the circuit board 51, a ground conductor 54 made of a conductive pattern is provided.
Further, on an inner layer of the circuit board 51, an L-shaped resonance strip line 55 made of a conductive pattern is provided. The resonance strip line 55 has connection portions 55a and 55b provided at both ends thereof.
thereof.
Furthermore, an entire back surface of the strip line 55 faces the ground conductor 54. In addition, the connection portion 55a of the strip line 55 is connected to the ground conductor 54 and the connection portion 55b thereof is connected to the wiring pattern 53.
Furthermore, on an inner layer of the circuit board 51 which is disposed between the wiring pattern 53 and the strip line 55, a ground pattern 56 made of a conductive pattern is provided. In addition, the ground pattern 56 has a conductor-deleted portion 56a which is provided at a position facing the surface of the strip line 55.
Further, the conductor-deleted portion 56a of the ground pattern 56 faces the entire surface of the strip line 55 and the ground pattern 56 is connected to the ground conductor 54 on the back surface of the circuit board 51.
Furthermore, on the surface 51a of the circuit board 51, various electronic parts 57 including a capacitor 57a and so on are mounted, such that a desired electrical circuit (oscillation circuit) is formed. When the electronic parts 57 are mounted on the circuit board 51, the capacitor 57a is connected to the connection portion 55b of the strip line 55 (for example, see Japanese Unexamined Patent Application Publication No. 5-136612).
Such a conventional voltage controlled oscillator has a configuration that the ground pattern 56 does not exist on the surface of the strip line 55 and the entire back surface of the strip line 55 faces the ground conductor 54, and thus reductions of inductance and Q are small. However, since the entire surface of the strip line 55 faces the surface 51a of the circuit board 51 through the conductor-deleted portion 56a, a shield characteristic between the strip line 55 and the wiring pattern 53 is deteriorated.
As described above, according to the conventional voltage controlled oscillator, since the entire surface of the strip line 55 faces the surface 51a of the circuit board 51 through the conductor-deleted portion 56a which is provided in the ground pattern 56, the shield characteristic between the strip line 55 and the wiring pattern 53 is deteriorated.